Nitride semiconductors such as GaN, AlN, and InN, or mixed crystals of these materials have wide band gaps, and are used as high-output electronic devices, short-wavelength light-emitting devices, and the like. To be used as high-output devices, technologies relating to field-effect transistors (FET), especially, high electron mobility transistors (HEMT), have been developed (see, for example, Patent Document 1). A HEMT that uses such a nitride semiconductor is used for a high-output, high-efficiency amplifier, a high-power switching device, or the like.
Incidentally, normally-off is required as one of the characteristics for a high-output, high-efficiency amplifier, a switching device, and the like. Also, normally-off is important from a safety operation point of view. However, in a HEMT using GaN, it is considered difficult to realize normally-off because the density of electrons is extremely high in 2DEG (Two-Dimensional Electron Gas) generated in an electron transit layer due to piezoelectric polarization and spontaneous polarization in GaN. Therefore, various methods are under investigation to realize normally-off for a HEMT using GaN.
One of the methods to realize normally-off for a HEMT is a method that forms a gate recess. Specifically, the method forms a recess in an electron supply layer immediately below a gate electrode to make 2DEG disappear in the region immediately below the gate electrode to realize normally-off.
Also, as one of the semiconductor devices using a nitride semiconductor, there is a transistor that has a UMOS (U Metal Oxide Semiconductor) structure in which a U-shaped opening is formed in stacked nitride semiconductor layers, and an oxide film is formed on the opening.